Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer program therefor.
Description of the Related Art
The electrophotographic printing method utilizes heat and pressure at a stage of development processing, which is processing for fixing a toner including a pigment onto a sheet. When subjected to heat and pressure, the sheet may be contracted or expanded.
For a sheet that can be used on an electrophotographic printing apparatus, the above-described phenomenon of sheet contraction or expansion may arise remarkably on the first fixing stage but only a substantially ignorable displacement may occur in the second and subsequent fixing stages.
A printed image may become contracted or expanded as the sheet having the image printed thereon becomes contracted or expanded. Accordingly, the size of an image printed on the front surface of the sheet may become slightly different from an image printed on the back surface of the sheet. In a field for which a high printing accuracy is considered significant, it is necessary to correct the above-described difference.
In order to solve the above-described issue, a conventional method implements a countermeasure against a contracted or expanded sheet by adjusting image information to be printed itself. More specifically, Japanese Patent Application Laid-Open No. 61-206365 discusses a method for enlarging or reducing image information to be printed by executing insertion or thinning (hereinafter collectively referred to as “insertion or removal processing”) of one pixel selected at random from among a predetermined number of pixels.
By executing insertion or removal processing at a pixel position determined at random, a halftone image can be enlarged or reduced with a high image quality by executing insertion or removal processing. However, if the position of a pixel of a halftone image on which the insertion or removal processing is to be executed (i.e., an insertion or removal position) is determined at random by using a random number, a dither pattern generated by dithering may be broken. As a result, when the image is printed, dots may become unstable.
Suppose that thinning is executed in a sub scanning direction at a position having coordinates (x, y) (x: main scanning position and y: sub scanning position). In this case, if thinning of once per one hundred lines is executed with a reduction rate of 1%, the random number can have the following values:Random(x): 0-99where the function Random(x) is a function for generating a random number based on the argument x.
Assuming that main scanning positions x0 and x1, at which thinning is to be executed, are located on mutually adjacent columns, that Random(x0)=2, and that Random(x1)=96, then pixels to be thinned off have coordinates (x, y)=(x0, y2), (x1, y96). To paraphrase this, as illustrated in FIG. 11A, the thinning positions are separate from each other by ninety-four lines in the sub scanning direction.
FIG. 11B illustrates a state in which designated pixels have been actually thinned off and the pixels are shifted as a result. At the main scanning position x0, the thinning is executed on the line y2. Accordingly, output positions on the line y3 and subsequent lines ascend in the sub scanning direction by one line. On the other hand, at the main scanning position x1, the line y96 is subjected to the thinning. Accordingly, output positions on the line y97 and subsequent lines ascend in the sub scanning direction by one line.
To paraphrase this, the lines y97 through y99 in the state illustrated in FIG. 11A are shifted to the lines y96 through y98 in the state illustrated in FIG. 11B only. Accordingly, the dither pattern between the main scanning positions x0 and x1 on the line y96 and subsequent lines (FIG. 11B) may not be broken. Furthermore, because the line y99 (a region surrounded by a broken-line rectangle illustrated in FIG. 11B) disappears, the image is successfully reduced by 1%.
On the other hand, for the lines y2 through y95, the main scanning positions x0 and x1, which exist adjacent to each other, become shifted away from each other due to thinning. Accordingly, the dither pattern may be generated in a broken state.
As illustrated in FIG. 11A, the pixels of different positions are thinned off on the main scanning positions x1 through x3. Accordingly, the pattern illustrated in FIG. 11B, which is a pattern after the thinning is completed, becomes different from the dither pattern before the thinning illustrated in FIG. 11A. As a result, image forming by the electrophotographic method may not be executed in a stable state because the image forming is executed based on the pattern different from the pattern before the thinning.